Laser-induced fluorescence measurements in lean direct-injection spray flames: technique development and application

The combustion diagnostics community has recently begun to focus its efforts toward practical combustion devices. One impetus behind this effort is the need to develop aeropropulsion gas turbine combustors with ultra-low NO_x emissions. For the past several years, the Flame Diagnostics Laboratory at Purdue University has been advancing optically non-intrusive techniques to measure concentrations of nitric oxide [NO] in lean direct-injection (LDI) spray flames. LDI flames offer the possibility of reducing NO_x emissions from gas turbines by rapid mixing of the liquid fuel and air so as to drive the flame structure toward partially premixed conditions. In this paper, we review the technical approach required to utilize laser-induced fluorescence (LIF) methods for quantitatively measuring [NO] in LDI spray flames. In the progression from atmospheric to high-pressure measurements, the LIF method requires a shift from the saturated to the linear regime of fluorescence measurements. As such, we discuss quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of NO concentration in atmospheric, LDI spray flames. In general, the results are comparable, although novel filtering techniques are required at higher flame pressures.